group by

data/3.Java高阶/5.分组/1.操作分组的元素/config.json

-{
-  "node_id": "java-4cc01469b31d42ac80854c4024c1b15a",
-  "keywords": [],
-  "children": [],
-  "export": [],
-  "title": "操作分组的元素"
-}
\ No newline at end of file

data/3.Java高阶/5.分组/2.多级分组/config.json

-{
-  "node_id": "java-873636226f0e44ad9269f2a13fe2adb9",
-  "keywords": [],
-  "children": [],
-  "export": [],
-  "title": "多级分组"
-}
\ No newline at end of file

data/3.Java高阶/5.分组/3.按子组收集数据/config.json

-{
-  "node_id": "java-1223956aa7ae47a9a2e517dc23c3f371",
-  "keywords": [],
-  "children": [],
-  "export": [],
-  "title": "按子组收集数据"
-}
\ No newline at end of file

data/3.Java高阶/5.分组/group_by.md

+# 分组
+
+已知类型
+
+```java
+public class Employee{
+    int id;
+    String name;
+    String department;
+}
+```
+
+将 `List<Employee> employee` 中的员工信息按部门分组，其中不正确的是：
+
+## 答案
+
+```java
+// List<Employee> employee = ...;
+Map<String, Object> result = employee.stream().collect(toMap(Employee::getDepartment, Function.identity()));
+```
+
+## 选项
+
+### 标准方法
+
+```java
+// List<Employee> employee = ...;
+Map<String, List<Employee>> result = employee.stream().collect(groupingBy(Employee::getDepartment));
+```
+
+### 朴素方法
+
+```java
+// List<Employee> employee = ...;
+Map<String, List<Employee>> result = new HashMap<>();
+for(var emp: employee){
+    var list = result.getOrDefault(emp.department, new ArrayList<>());
+    list.add(emp);
+}
+```
+
+### 朴素方法
+
+```java
+Map<String, List<Employee>> result = new HashMap<>();
+for(int index=0; index < employee.size(); index++){
+    var emp = employee.get(index);
+    var list = result.getOrDefault(emp.department, new ArrayList<>());
+    list.add(emp);
+}
+```